1. Field of the Invention
The present invention relates to a method for selecting a knee prosthesis and, more precisely, to a method of selection for choosing one or more elements of a knee prosthesis such as, in particular, a prosthetic femoral and tibial implant and/or a tibial or femoral wedge, from an available set of elements.
The method may also allow determination, on a computer model of the patient's knee, of resection planes, in particular femoral and tibial resection planes, intended to serve as a seat for the corresponding portion of the knee prosthesis.
The invention also relates to a device for carrying out this method.
2. Description of the Related Art
In conventional operations for inserting knee prostheses, the surgeon makes tibial and femoral osseous cuts depending on the patient's anatomical characteristics and the type of commercially available prosthesis, then, during insertion, makes adjustments, for example using wedges or even by remaking a resection incision to optimise as far as possible the articular properties of the prosthesis when it is in operation.
It will be appreciated that this optimisation depends greatly on the expertise of the surgeon and the anatomical features of the knee to be operated on.
The objective sought is to obtain, if possible, equal tension in the soft portions of the knee at 0 and 90° which are maintained over the entire arc of flexion of the prosthesis, satisfactory geometric alignment and extension without flexum to optimise the stresses in the standing position and obtain the most appropriate result for the patient's anatomy. A significant objective is to obtain good stability of the knee by appropriate equilibrium of the ligaments.
For this purpose, it has already been proposed that the surgeon be assisted by computerised measuring means and measurement data processing means.
It is accordingly known to store the anatomical configuration of the distal end of the patient's femur and the proximal end of the tibia on the basis of measurement data obtained by any means. This data may be obtained, for example, by scanning or preferably by in situ measurement. It is possible to use, in a three-dimensional spatial reference system defined using reference markers (for example, reflective reference beads), infrared or magnetic rays fixed at three suitable positions on a knee epiphysis, by displacement of a probe which is also marked in space by the acquisition means, software of a known type for reconstituting the precise three-dimensional shape of the ends in question. More precisely, a device of this type comprises a locating transceiver such as a high-definition infrared camera for marking fixed reference points on the patient and the marked instruments used, such as the pointer or probe, cutting guide, etc., storage and calculating means such as a computer employing 3D type modelling software, preferably a display means such as a screen, and a control means such as a mouse or tactile screen or preferably a pedal actuated by the surgeon's foot.
Suitable marking on the portion of the knee which moves relative to the reference system also allows calculation of the relative positions of the femur and the tibia.
It is thus known, after resection of the tibial plate and optionally insertion of a prosthetic tibial plate supporting element, and while using a tensor introduced by the surgeon into the space between the tibial end and the femoral articular end, to determine, under the chosen tension value imposed by the tensor, the distance between the tibia and the femur as well as the HKA angle, in other words the angle, taken internally, between the femoral mechanical axis (defined by the centre of the hip and the centre of the knee) and the tibial mechanical axis (defined by the centre of the knee and the centre of the ankle), on the one hand, when extended or in a position as close as possible to extension and, on the other hand, when flexed at 90°, the surgeon then choosing the most suitable constituent prosthetic elements from the set of available elements, this choice being able to be displayed on the screen prior to insertion, by modelling the position of the preselected element of which the characteristics have been stored in the computer.
It is noted, however, that this technique does not always allow optimal choice and/or positioning of the selected prosthetic element(s), and this prevents optimum biomechanics, particularly during retraction of the soft posterior portions of the knee in flexum and in the phases of intermediate flexion between 0 and 90° and beyond 100°. The optimum biomechanics correspond to “good tension” of the soft portions over the entire sector of movement, namely stability tension for the supporting zones and micro-play of laxity between 20 and 140°, allowing easy mobility without hypertension or uneven or exaggerated laxity.